1. Field of the Invention
This invention relates to the field of transmission line drivers, and particularly to circuits and methods for driving capacitively-terminated transmission lines.
2. Description of the Related Art
Transmission lines are used to convey high frequency digital data signals between a source circuit and a destination circuit. The destination circuit can be considered a load, which typically includes a capacitive component. Transmission lines having a capacitive load at their receiving end are referred to herein as capacitively-terminated transmission lines.
A transmission line has a characteristic impedance Z0, as does the load being driven. Ideally, the impedances of the load and the transmission line are equal; when so arranged, a data signal transition that occurs at the source end of the transmission line is fully absorbed by the load, with none of the energy reflected back towards the source by the load. However, it is often impossible to make the transmission line and load impedances equal. For example, in digital circuits the load is often a transistor gate whose impedance is almost purely capacitive. A capacitive load causes all of the incident energy to be reflected back toward the source in a negative voltage wave. In some instances, a load resistor is added to the capacitive load in an attempt to properly terminate the transmission line. However, the capacitance at the end of the transmission line shunts out the load resistance at higher frequencies, resulting in an impedance mismatch; consequently, with a capacitive load impedance the data signal transition is at least partially reflected back towards the source. When this negative wave propagates to the source end of the transmission line, it may again be reflected back towards the load, thereby distorting the data signal being conveyed. The output impedance of the source circuit is typically much lower than Z0, which allows for a large pulse amplitude to be delivered to the transmission line. This creates a fast transition slew rate at the capacitively-terminated end of the transmission line. However, a low output impedance tends to maximize the magnitude of the signal reflected by the source back towards the load, and thus the data signal distortion. The above principle is applicable to any complicated capacitively-terminated transmission line.
The problem noted above tends to become more acute as the frequency of the transitions increases; i.e., data signal distortion is less for a data bit pattern of 111000 than it is for a pattern of 101010. “Waveshaping” the data bit signal is sometimes used to mitigate this problem. One such approach requires looking ahead at a predetermined number of upcoming data bits, and increasing the amplitude of the transmitted signal when the transition frequency increases. However, this approach requires complex look-ahead and output voltage adjustment circuitry, which does nothing to reduce the magnitude of the reflected waves, or to address their causes.